Kinematics Flashcards
Explain the physics behind a lift going up
The equation of motion here is T-W=ma.
1) We start going up. There is tension in the cables upwards and the weight of the lifts and the passengers is acting downwards. T>W so T-W=ma, and T-W is positive which shows a=positive, so we are accelerating.
2) T-W=ma. But now T=W. So T-W=0. So acceleration = 0. At this instant, we have reached terminal velocity, and the acceleration is 0, and the resultant force is 0 and the velocity is constant.
3) Now W>T. T-W=ma. As W>T ma is negative. This means the lift starts to decelerate as we slow down and then the lift eventually comes to a stop.
Explain the physics behind a lift going down.
W-T=ma is our equation of motion.
1) Initially W>T so ma is positive, which shows acceleration is positive, and at first we accelerate downwards.
2) Then W=T, so W-T = 0, and so ma is also 0, mass can’t be 0 so the acceleration is 0. We have reached terminal velocity. The acceleration is 0, so is the resultant force, and the velocity is constant.
3) T>W. Ma = negative. We are decelerating and eventually we come to a stop.
Explain the physics behind the force the passenger feels while going up in a lift.
The equation of motion here is N-W=ma.
1) N=ma+W. Acceleration is positive initially, as we accelerate. We feel our normal contacts force and we feel heavier.
2) N=ma+W. At terminal velocity, the acceleration is 0. This is mean N=W. We feel our weight.
3) We are about to stop, and now, we start decelerating. N=ma+W. Accelerating in negative, so there is something getting subtracted from W, so we feel lighter.
Explain the physics behind the forces the passenger feels in a light going down.
The equation of motion is W-N=ma.
1) N = W-ma. We feel out normal contact force. Initially, acceleration is positive, so we feel lighter.
2) N= W-ma. Then acceleration is 0 at terminal velocity, so we feel our weight.
3) N=W-ma. Then acceleration is negative. 2 negatives make a plus, so we feel heavier.
How do we calculate the braking distance ?
We use the equation v^2 = u^2+2as. We are braking so v, our final velocity will be 0. Rearrange for s and we get s = -u^2 / 2a. Our acceleration should be deceleration so a should be negative and our overall distance would be positive.
How do we calculate braking distance?
We use the equation v^2 = u^2+2as. We are braking so v, our final velocity will be 0. Rearrange for s and we get s = -u^2 / 2a. Our acceleration should be deceleration so a should be negative and our overall distance would be positive.
We use the equation v^2 = u^2+2as. We are braking so v, our final velocity will be 0. Rearrange for s and we get s = -u^2 / 2a. Our acceleration should be deceleration so a should be negative and our overall distance would be positive.
How do we calculate thinking distance?
Reaction time x speed. From the equation speed=distance/time.
How do we calculate stopping distance ?
Stopping distance = thinking distance + braking distance.
= -u^2 / 2a + ut
How do we calculate impact force, and how can we reduce the impact force perhaps during an accident?
V = u + at.
A = v-u / t.
F=ma.
F= m(v-u / t) This is the equation for impact force.
From the equation we can see that to reduce the impact force, we need to increase the impact time. We can do this by crumple zones, air bags, seatbelts. We could also reduce speed.
We can find ways to increase the impact time from an equation.
S=1/2(u+v)(t)
T = 2s - (u+v)
To reduce the time, we want a large impact distance.
What is Newton’s first law of motion?
An object will stay stationary or moving at constant velocity until a force is applied.
This shows how a force is needed to change the momentum of an object .
What is Newtons 2nd law of motion?
Newtons 2nd law of motion is that the rate of change of momentum of an object is directly proportional to the force acted on it.
F=ma where acceleration can be written as v-u/t
What is the impulse of a force?
This is the force x time for which the force acts.
It is equal to the change of momentum of an object.
Impulse = F x delta T
Impulse = delta p (momentum) = mv (final) - mu (initial)
Impulse is a vector quantity and can be found by working out the area under a force - time graph.
When an object hits a boundary at an angle, how do we work out the impact?
We are interested in the component of the objects velocity that hits the boundary, perpendicular to the boundary. So we need to split the velocity into components and use the component of the objects velocity that is perpendicular to the boundary.
The change in momentum is found using final - initial.
Ensure to use directions.
What is newtons 3rd law of motion?
When two objects interacts, they each exert equal and opposer forces on each other.
What is the principle of conservation of momentum?
The total momentum of a closed system of interacting objects re,mains constant, provided that no external forces are acting on it.
What is an elastic collision?
An elastic collision is one where the kinetic energy remains constant, and there is no loss of kinetic energy.
What is an inelastic collision?
An inelastic collision is one where there is a loss of kinetic energy. The kinetic energy of the colliding objects AFTER the collision is less than the kinetic energy of the objects BEFORE the collision.
Some energy is transferred to the surroundings.
What is the principle of conservation of energy ?
Energy cannot be created or destroyed. Whenever energy is transferred, the total amount of energy before the transfer is equal to the total amount of energy after the transfer.
Describe forces at work
Work is done when a force acting on it makes it move. W=Fs. Remember the force here must e in the direction of movement of the object.
If it is at angle, resolve the force and use the component that is parallel to the movement of the object.
Define power
Power is the rate of energy transfer.
It can be calculated using energy/time or work done/time
How can we work out the power of an object moving at constant velocity driven by a constant force?
Power = force x velocity
This force must be in the direction of the velocity.
